Ensure the Blackberry Desktop Manager is installed using Blackberry Internet Service, and not Blackberry Enterprise Server. If you are unsure, it would probably be a good idea to uninstall the Desktop Manager and start again.
If you don't have the CD that came with your Blackberry, the Software can be downloaded here. BlackBerry Software Site BlackBerry - Smartphone Software - BlackBerry Applications - Smartphone Downloads
Step 2

Wipe your Blackberry, creating a backup if necessary. Select Options/Security/Wipe (Newer 4.2 OS Devices Goto Options/Security Options/General Settings/Wipe) on the Device.
If this option is unavailable, you may have to install the latest software on your Blackberry. You need to Download and install the latest Desktop Manger Software, then the latest Handheld Software. Connect your device, open the Desktop Manager, select Application Loader, and follow the prompts.
Step 4

Close the Desktop Manager if it is open.
Step 5

From the Windows Start Menu select Run..., and at the prompt type regedit. In the tree on the left hand side, navigate to:
HKEY_Current_Users\Software\Research In Motion\BlackBerry\PolicyManager
Right-Click the Policy Manager Folder and select New/String Value. Name the value Path. Now, Double-Click the Path Subkey and set Value Data to:
C:\Program Files\Research In Motion\BlackBerry\policy.bin
Step 6

Open the Desktop Manager.
Step 7

Connect the Device.

Verification

Once complete, the Options/Security screen on your Blackberry should not contain references to an IT Policy, you should now be able to change all settings (including password prompts), and install Third Party Applications.

It should be stressed that this is NOT for BlackBerrys currently on a BES, only for those that were once on a BES and were retired to personal use. Circumventing IT Policies is often cause for termination.